On Mon, Apr 6, 2020 at 3:38 PM Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > On Mon, Apr 06, 2020 at 02:32:51PM +0200, Daniel Vetter wrote: > > On Fri, Apr 3, 2020 at 3:58 PM Daniel Vetter <daniel.vetter@xxxxxxxx> wrote: > > > > > > In drm we've added nice drm_device (the main gpu driver thing, which > > > also represents the userspace interfaces and has everything else > > > dangling off it) init functions using devres, devm_drm_dev_init and > > > soon devm_drm_dev_alloc (this patch series adds that). > > > > > > A slight trouble is that drm_device itself holds a reference on the > > > struct device it's sitting on top (for sysfs links and dmesg debug and > > > lots of other things), so there's a reference loop. For real drivers > > > this is broken at remove/unplug time, where all devres resources are > > > released device_release_driver(), before the final device reference is > > > dropped. So far so good. > > > > > > There's 2 exceptions: > > > - drm/vkms|vgem: Virtual drivers for which we create a fake/virtual > > > platform device to make them look more like normal devices to > > > userspace. These aren't drivers in the driver model sense, we simple > > > create a platform_device and register it. > > > > > > - drm/i915/selftests, where we create minimal mock devices, and again > > > the selftests aren't proper drivers in the driver model sense. > > > > > > For these two cases the reference loop isn't broken, because devres is > > > only cleaned up when the last device reference is dropped. But that's > > > not happening, because the drm_device holds that last struct device > > > reference. > > > > > > Thus far this wasn't a problem since the above cases simply > > > hand-rolled their cleanup code. But I want to convert all drivers over > > > to the devm_ versions, hence it would be really nice if these > > > virtual/fake/mock uses-cases could also be managed with devres > > > cleanup. > > > > > > I see three possible approaches: > > > > Restarting this at the top level, because the discussion thus far just > > ended in a long "you're doing it wrong", despite that I think we're > > doing what v4l is doing (plus/minus that we can't do an exact matching > > handling in drm because our uapi has a lot more warts, which we can't > > change because no breaking userspace). > > > > So which one of the three below is the right approach? > > > > Aside, looking at the v4l solution I think there's also a confusion > > about struct device representing a char device (which v4l directly > > uses as its userspace interface refcounted thing, and which drm does > > _not_ directly). And a struct device embedded into something like > > platform_device or a virtual device, where a driver can bind to. My > > question here is about the former, I don't care how cdev struct device > > are cleaned up one bit. Now if other subsystems relies on the devres > > cleanup behaviour we currently have because of such cdev usage, then > > yeah first approach doesn't work (and I have a big surprised that use > > case, but hey would actually learn something). > > > > End of aside, since again I want to figure out which of the tree > > approaches it the right one. Not about how wrong one of them is, > > ignoring the other three I laid out. And maybe there's even more > > options for this. > > Sorry, been swamped with other things, give me a few days to get back to > this, I need to dig into how you all are dealing with the virtual > drivers. Sure, no problem. > Doing this in the middle of the merge window is a bit rough :) Ah I always forget ... we freeze drm at -rc6, so merge window is actually my most relaxed time since everyone is busy and no one has time to report drm bugs :-) Thanks, Daniel -- Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
